Necessity of Ga prelayers in GaAs/Ge growth using gas-source molecular beam epitaxy

We have investigated the gas-source molecular beam epitaxy growth of GaAs on a (001) Ge surface, off-cut 6° towards the [110]. Initiation of GaAs on Ge with the typical procedure of using a self-terminating As layer invariably produces poor GaAs surface morphology and generates a plethora of antiphase boundaries, despite the large miscut of the (001) surface. However, initiation of GaAs growth with approximately 1 ML (monolayer) of Ga results in single-domain films with excellent surface morphology. We conclude that the Ga monolayer prevents the Ge surface from forming a high step density surface which is established when Ge is exposed to As2.